The invention relates to controlling curvature in micro-structures. More particularly, the present invention relates to controlling curvature in micro-structures used in micro-electro mechanical machines.
A number of micro-machines utilize movable cantilever or ribbon structures. Typically the cantilever or ribbon structures are extremely thin; on the order of hundreds or thousands of Angstroms. These thin micro-structures can experience a high degree of mechanical stress, either during fabrication or during use, which can cause the micro-structures to permanently buckle or bend. The resultant curvatures can reduce the efficiency and/or functionality of the micro-device, especially when the cantilever or ribbon structure is configured to reflect light, such as in the case of a optical MEM device.
One type of optical MEMS that is used to modulate one or more wavelengths of light is disclosed in the U.S. Pat. No. 5,311,360, issued to Bloom et al., which is hereby incorporated by reference. More advanced designs and techniques for making optical MEM devices are described in the U.S. Pat. No. 5,841,579 and the U.S. Pat. No. 5,808,797, both issued to Bloom et al., the contents of which are also both hereby incorporated by reference.
Briefly, optical MEM devices described in the above referenced patent have one or more sets of movable ribbons that comprise a support layer and a reflective top-layer. The support layer is preferably silicon nitride and the reflective top layer is preferably aluminum. Because of the high degree of stress between support layer and the reflective top-layer, the ribbons tend to buckle which significantly reduce the available optical surface area and hence the efficiency of the optical MEM device. Therefore, there is a need for a method of fabricating substantially flat micro structures, such as ribbons and/or cantilevers.
The current invention is directed to a micro-device comprising at least one suspended micro-structure which is preferably a ribbon structure or cantilever structure, also referred to herein as a release structure. The release structure comprises a silicon-based support layer which is preferably a silicon nitride layer and a reflective top layer that is preferably aluminum. The micro-device is preferably configured to modulate a light with the release structure.
In accordance with the embodiments of the invention, a stress compensating silicon dioxide layer is positioned between the support layer and the top layer. The stress compensating layer helps to control or reduce curvature of the micro structure or release structure.
In a preferred embodiment of the invention, the micro device is an optical MEM which comprises a release structure with plurality of ribbon structures. In accordance with the embodiments of the invention, each of the ribbons has an average width in a range of about 0.5 to about 13 microns and average length in a range of about 10 to about 800 microns. Preferably, alternating ribbons are configured to be selectively moved, relative to stationary ribbons, in order to modulate light.
Each of the ribbons preferably has a silicon-nitride support layer with a thickness in a range of about 800 to about 1200 Angstroms, a reflective top layer with a thickness in a range of about 500 to about 1000 Angstroms and a silicon dioxide compensating layer with a thickness in a range of about 800 to about 8000 Angstroms. Each ribbon can be an individual ribbon or, alternatively, the ribbons are formed as a monolithic comb structure. Regardless of the configuration, the ribbons are preferably coupled to and secured to a support substrate through one or more securing features. The support substrate is preferably a silicon-based wafer.
In accordance with the embodiments of the invention, a compensating layer is selectively formed over regions corresponding to. active regions of the ribbons. Further, the compensating layer can be selectively formed over the central portions of the active regions of the ribbons. In accordance with the embodiments of the invention, the compensating layer thickness can be controlled, such that the height difference between portions of a ribbon with the compensating layer and the portions of the ribbon without the compensating layer corresponds to a distance approximately equal to nxcex/2 (wherein n=a whole number). Accordingly, light that is reflected from portions of the ribbon with the compensating layer and the portions of the ribbon without the compensating layer will be in phase. In yet further embodiments, ribbons or a micro-structures with compensating layers, such as described above, are annealed to further reduce the curvature as described below.
In a preferred embodiment of the invention, a plurality of movable ribbons with compensating layers are encapsulated within a die structure and are electrically coupled to a driver circuit for selectively moving alternating ribbons.
In accordance with a preferred method of the invention, a micro-device is formed from a substrate comprising a device layer, wherein the device layer comprises a silicon-nitride under-layer, a reflective top-layer and a stress compensating silicon dioxide layer therebetween. The release features of the micro-device are patterned from a device layer using a lithographic mask and etch techniques, wherein portions of the underlying substrate are etched away to release the patterned device layer.
The support features for supporting the micro-structures on the substrate, are preferably formed by etching trenches or dimples into the substrate before forming or depositing the device layer. The device layer then fills the trenches or dimples and integrates the device layer into the substrate such that the micro-structures remain attached to the substrate after a subsequent etching process.